I don’t really know of a way that you can add insulation without taking up interior space, exterior space, or replacing the wall with more modern materials.

• What about the sides of the home not facing the street? Can you add an exterior layer of insulation and then new exterior siding to those walls?
• On the side facing the street, you could replace the stone wall with a different type of wall that was more thermally resistant. This would of course be a major undertaking.
• I am sure you have considered fully insulating the floor and ceiling as best you can.
• If rodents and or insects are a concern, look into Mineral Wool/Rockwool Insulation.